Decorative Plating

The traditional gold electroplating solution (Table 1) for decorative use required:

A source of gold

• A complexing agent for the gold

• A conducting salt to help carry the current and broaden the conditions of operation

• An alloying metal or metals for color and/or hardness

The source of gold was historically gold cyanide. The complexing agent was sodium or potassium cyanide (Table 1). The conducting salts were cyanides, phosphates, carbonates, hydroxides, and occasionally but rarely citrates, tartrates, and so forth.

Table 1 Typical flash formulations for decorative gold plating

Component or parameter

Type of jewelry plating

English (24K)

Hard (18K)

Hamilton(a)

White

Rose

Green

Barrel flash

Gold as potassium gold cyanide, g/L (oz/gal)

2 (0.3)

1.6 (0.2)

1.25 (0.15)

0.4 (0.05)

4.1 (0.5)

2 (0.3)

0.8 (0.1)

Free potassium cyanide, g/L (oz/gal)

7.5 (1)

7.5 (1)

7.5 (1)

15 (2)

3.75 (0.5)

7.5 (1)

7.5 (1)

Dipotassium phosphate, g/L (oz/gal)

15-30 (2-4)

15-30 (2-4)

15-30 (2-4)

15-30 (2-4)

15-30 (2-4)

60-90 (812)

Sodium hydroxide, g/L (oz/gal)

15 (2)

Sodium carbonate, g/L (oz/gal)

30 (4)

Nickel as potassium nickel cyanide,g/L (oz/gal)

0.15-1.5 (0.02-0.2)

0.3 (0.04)

(0.15)

0.3 (0.04)

Copper as potassium copper cyanide, g/L (oz/gal)

1.5 (0.2)

Silver as potassium silver cyanide, ppm

200

Temperature, °C (°F)

60-70 (140158)

60-70 (140158)

65-70 (150158)

65-82 (150-180)

54-65 (130150)

49-60 (120140)

Current density, A/dm2 (A/ft2)

1-4 (10-40)

1-4 (10-40)

1-3 (10-30)

2-5.5 (2055)

1-2 (10-20)

0.5-10 (510)

(a) Hamilton is a term that has been applied to white, pink, green, and brown golds. It is practically meaningless today, but is still widely used.

(a) Hamilton is a term that has been applied to white, pink, green, and brown golds. It is practically meaningless today, but is still widely used.

If any four numbers are randomly assigned to the concentrations of the four constituents of the gold electroplating solution, plating conditions can be found that will yield a satisfactory deposit. The four numbers chosen would determine the necessary temperature of operation, the degree of agitation, the current density for producing a good deposit, and the time of plating needed for different thicknesses. The fact that any four numbers could be used explains why hundreds of formulations appear in the literature. Given the proper operation conditions, any of the formulas will work, and at one time or another each cited formula was optimum and economic for a given plant and a given plater. Variations in the price of gold, the size of the item to be plated, the necessary rate of production, the desired deposit thickness, and the desired color resulted in almost every plater designing the "best bath."

Today, most jewelry is flash plated or strike plated from a hot-cyanide alloy (color) bath. The deposit is usually applied over a bright nickel deposit. Occasionally, the gold is flash plated over a palladium deposit over a bright acid-copper deposit, where nickel-free deposits are desired. (The European Common Market is concerned about nickel dermatitis from costume jewelry, snap fasteners, and other items that contact the skin.) Occasionally, the flash gold deposit is applied over a karat gold or rolled-gold plated item. This is done to give an even color to jewelry items made of several different findings. (Some jewelry is flashed from an acid bath directly over stainless steel for hypoallergenic jewelry.)

Typical flash formulations are given in Table 1. Although broad ranges are given for the decorative flash baths, it is absolutely essential that each parameter be closely and tightly controlled within its range if consistency of color is desired. The time of plating is quite short, usually 5 to 30 s. For minimum porosity and subtle color matches, even a 30 s plate may be duplex plated from two different solutions. For flash barrel plating the gold concentration can be as low as 0.8 g/L, the free cyanide is 7.5 g/L, the dipotassium phosphate should be 75 g/L or above, and nickel, as a brightener, should be added at 2 g/L or higher as potassium nickel cyanide.

The deposit is generally 0.05 to 0.1 pm (2 to 4 pin.) and cannotbe marketed as gold electroplate. If the jewelry is to be marketed as gold electroplate the deposit must be 0.175 pm (7 pin.). If the jewelry is to be marketed as heavy gold electroplate the deposit must be 2.5 pm (100 pin.). Most deposits in this range are plated from an acid gold formulation (Table 2) or from a sulfite gold bath (Table 3).

Table 2 Acid gold color plating baths for heavy deposits

Component or parameter

1N Color(a)

2N Color(a)

Yellow 24K

Yellow 22K

Gold, g/L (oz/gal)

0.4-0.8 (0.05-0.1)

0.4-0.8 (0.05-0.1)

0.4-0.8 (0.05-0.1)

0.4-0.8 (0.05-0.1)

Conducting salt(b), g/L (oz/gal)

120 (16)

120 (16)

120 (16)

120 (16)

Nickel as chelate, g/L (oz/gal)

11 (15)

3.7-6 (0.5-0.8)

200 ppm

Cobalt as chelate, ppm

250

1000

pH

4-4.5

4-4.5

4.4-4.8

4.5

Temperature, °C (°F)

50-60 (120-140)

38-50 (100-120)

26-32 (80-90)

32-38 (90-100)

Current density, A/dm2 (A/ft2)

1-2 (9-19)

1-2 (9-19)

1-2 (9-19)

1-2 (9-19)

Agitation

Yes

Yes

Yes

Yes

(a) European color standards.

(a) European color standards.

(b) The conducting salt can be a phosphate or an organic acid such as citric or malic.

Table 3 Sulfite gold decorative plating baths

Component or parameter

24K

Flash green

Pink

Heavy plating

Gold as sulfite, g/L (oz/gal)

1.25-2 (0.17-0.27)

1.25-2 (0.17-0.27)

1.25-2 (0.17-0.27)

8-12 (1.0-1.6)

Conducting sulfite salt, g/L (oz/gal)

90 (12)

90 (12)

90 (12)

45-75 (6-10)

Nickel as chelate, g/L (oz/gal)

1.1 (0.15)

0.5 (0.07)

Copper as chelate, g/L (oz/gal)

0.5 (0.07)

Cadmium as chelate, ppm

760

Brightener, often arsenic, ppm

20

20

20

20

Current density, A/dm2 (A/ft2)

3-5 (28-46)

3-5 (28-46)

3-5 (28-46)

0.1-0.4 (1-4)

Temperature, °C (°F)

50-65 (120-150)

50-65 (120-150)

50-65 (120-150)

50-60 (120-140)

Time, s

10-20

15-30

10-20

(a)

As with cyanide gold plating, to achieve consistent good color control it is necessary to regulate each chemical and physical variable within its range given in Table 2. It is also necessary to analyze for metallic impurities and control their concentrations. Drag-in of metallic impurities can have a disastrous effect on color control.

Sulfite gold plating solutions (Table 3) have several unique and advantageous characteristics. First, they contain no cyanide, so the normal safety precautions used when working with or handling cyanide are not necessary when using sulfite gold. In addition, of course, there is no cyanide to destroy in the dragout, rinse stream or old solutions shipped for recovery. The second unique property is exceptional microthrowing power; the bath will actually build brightness during plating. The deposit is essentially featureless with exceptionally fine crystal structure.

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